Hair generally can be straight, wavy, curly, kinky or twisted. A human hair includes three main morphological components, the cuticle (a thin, outer-most shell of several concentric layers), the cortex (the main body of the hair), and, in case of higher diameter hair, the medulla (a thin, central core). The cuticle and cortex provide the hair strand's mechanical properties, that is, its tendency to have a wave, curl, or kink. A straight hair strand can resemble a rod with a circular cross-section, a wavy hair strand can appear compressed into an oval cross-section, a curly strand can appear further compressed into an elongated ellipse cross-section, and a kinky hair strand cross-section can be flatter still. The primary component of hair is the cross-linked, α-helix protein keratin. Keratins are intermediate filament proteins found specifically in epithelial cells, e.g. human skin and hair, wool, feathers, and nails. The α-helical type I and II keratin intermediate filament proteins (KIFs) with molecular weights around 45-60 kDa are embedded in an amorphous matrix of keratin-associated proteins (KAPs) with molecular weights between 20 to 30 kDa (M. A. Rogers, L. Langbein, S. Praetzel-Wunder, H. Winter, J. Schweizer, J. Int Rev Cytol. 2006; 251:209-6); both intra- and intermolecular disulfide bonds provided by cystines contribute to the cytoskeletal protein network maintaining the cellular scaffolding. In addition to the disulfide cross-links ionic bonding or salt bridges which pair various amino acids found in the hair proteins contribute to the hair strand's outward shape.
It is well known in the art that amino-acid based fibers, particularly hair, can be treated with functionalized silicones which deliver one or more cosmetic benefits, such as conditioning, color retention, shine and UV protection. Typically, these silicones are physically deposited on the fiber surface (cuticle) and therefore responsible for the outward appearance of the hair. They can be removed partially or completely by repeated washing processes. While the deposited silicones considerably improve the surface properties of the amino acid based fibers, i.e. smoothness and friction, they do not substantially impact the mechanical properties and the shape of the fibers.
People with naturally wavy, curly, or kinky hair may desire to reduce fizz and to get more control and a smoother appearance of their hairs. Several hair treatments including straightening methods are available, but these often involve the use of harsh and regulated substances.
Frequently used straightening preparations are based of sodium or potassium hydroxide blended with starch which are highly irritating to the scalp. Less irritating formulations are based on guanidine hydroxide or certain sulfites. Recent formulations are based on thio glycolic acid salts. The underlying principle is that under alkaline conditions disulfide bonds within the hair proteins undergo a reductive cleavage. The disulfide bonds are reduced to sulfhydryls and after shaping the desired hair configuration re-established by oxidation.
EP 295780, GB 1182939 and US 2012-0031420 propose to incorporate mercaptosilicones in this reduction/oxidation cycle. Target is a permanent conditioning effect. Disadvantages are the limited stability of the —SH moiety accompanied by an odor issue and the poor compatibility of these silicones with the aqueous carrier system.
The U.S. Pat. No. 5,679,619 proposes cystine modified silicones in combination with thioglycolates as reducing agent.
The U.S. Pat. No. 6,544,499 proposes silicones bearing protected —SH groups. Some of these protected systems are classified as being directly reactive towards —S—S— bonds whereas others need a reduction to —SH moieties. Disadvantage is the difficult synthesis of the protected structures.
The U.S. Pat. No. 5,935,560 describes thio and amino groups containing silicones which impart durable conditioning properties.
The different processes for straightening hairs have the tendency to weaken the strength of the hair. Therefore it is desirable to find compositions which enable straightening and recover strength and elasticity of the hair. Traditionally, aldehyde based formulations for a permanent hair shaping were developed. Most frequently formaldehyde is used for this so called Brazilian keratin shaping method (US 2012-0031420). The underlying principle is the crosslinking reaction between formaldehyde and keratin based amino and amido groups (H. Puchtler, Histochemistry, 82(1985), pp. 201-204) or between formaldehyde and —SH groups (US 2009-0211593) after straightening.
Dialdehydes, i.e. glyoxal, were proposed to replace formaldehyde (US 2009-0165812).
Silicones containing aldehyde functions are also proposed for personal care applications (US 2009-0137764), as cosmetic additive (US 2008-0161500) and more specifically for the permanent treatment of keratin fibers (US 2007-0129520). A disadvantage is the difficult synthetic process.
Methylol groups containing silicones were proposed for the treatment of cellulose based fibers (U.S. Pat. No. 3,432,536). In this case the polymer has the structure silicone-spacer-C(O)—NH—CH2OH. Methylol groups containing silicones of the structure silicone-spacer-N(CH2OH)—C(O)—R are proposed for the treatment of proteinacious fibers, i.e. hair (U.S. Pat. No. 8,110,648).
The U.S. Pat. No. 2,770,631 describes gallic acid modified silicones for sun screen and paint formulations. Resorcinol modified silicones were proposed as additives for polycarbonates in EP 2032624, EP 1951815 and as thermoplastic materials in GB 722822. WO 2007-078735 discloses the use of thioesters in rubber compounds.
DE 1111638 describes the synthesis of low molecular weight azetidinium compounds. Polymeric azetidinium derivatives are also known (H. Keul et. al., Macromolecules, 46, 638-646). Hydrocarbon based azetidinium compounds are used for the finishing of wool in the so called Hercosett process (EP 0236896).
None of the above prior art disclosures describes a straight forward synthetic methodology yielding stable and easy to formulate silicones which provide a benefit in a process for hair straightening with respect to a permanent hair shaping and hair strength without the usage of strongly irritating auxiliaries.